The present invention relates to a non-aqueous electrolyte secondary battery (hereinafter, battery). The present invention especially relates to batteries of which electrochemical properties such as charge/discharge capacity and charge/discharge cycle life have been enhanced by improvements in the negative electrode materials, separators and the amounts of electrolyte. The present invention further relates to batteries wherein the electrochemical properties mentioned above, as well as shelf stability, have been improved by designing a better balance between the positive electrode and the negative electrode materials, as well as the positive electrode and the negative electrode plates.
Lithium secondary batteries with non-aqueous electrolytes, which are used in such areas as mobile communications devices, including portable information terminals and portable electronic devices, as power sources of portable electronic devices, domestic portable electricity storing devices, motor cycles using an electric motor as a driving source, electric cars and hybrid electric cars, have characteristics of a high electromotive force and a high energy density. Although the energy density of the lithium secondary batteries using lithium metal as a negative electrode material is high, there is a possibility that dendrite deposits form on the negative electrode during charging. By repeated charging and discharging, the dendrite breaks through separators to the positive electrode side, thereby causing an internal short circuit. The deposited dendrite has a large specific surface area, thus its reaction activity is high. Therefore, it reacts with solvents in the electrolyte solution on its surface and forms a surface layer which acts like a solid electrolyte having no electronic conduction. This raises the internal resistance of the batteries or causes some particles to be excluded from the network of electronic conduction, lowering the charge/discharge efficiency of the battery. Due to these reasons, the lithium secondary batteries using lithium metal as a negative electrode material have a low reliability and a short cycle life.
Nowadays, lithium secondary batteries which use carbon materials capable of intercalating and de-intercalating lithium ions as a negative electrode material are commercially available. In general, lithium metal does not deposit on carbon negative electrodes. Thus, in such batteries short circuits do not occur due to dendrite formation. However, the theoretical capacity of graphite, which is one of the currently available carbon materials, is 372 mAh/g, only one tenth of that of pure lithium (Li) metal.
Other known negative electrode materials include pure metallic materials and pure non-metallic materials which form composites with lithium. For example, composition formulae of compounds of tin (Sn), silicon (Si) and zinc (Zn) with the maximum amount of lithium are Li22Sn5, Li22Si5, and LiZn respectively. Within the range of these composition formulae, metallic lithium does not normally deposit to form dendrites. Thus, an internal short circuit due to dendrite formation does not occur. Furthermore, electrochemical capacities between these compounds and each element in pure form mentioned above is respectively 993 mAh/g, 4199 mAh/g and 410 mAh/g; all larger than the theoretical capacity of graphite.
As an example of other compound negative electrode materials, the Japanese Patent Laid-Open Publication No. H07-240201 discloses a non-metallic siliside comprising transition elements. The Japanese Patent Laid-Open Publication No. H09-63651 discloses negative electrode materials which are made of inter-metallic compounds comprising at least one of group 4B elements, phosphorus (P) and antimony (Sb), and have a crystal structure of one of the CaF2 type, the ZnS type and the AlLiSi type.
However, the foregoing high-capacity negative electrode materials have the following problems. Negative electrode materials of pure metallic materials and pure non-metallic materials which form compounds with lithium have inferior charge/discharge cycle properties compared with carbon negative electrode materials. The reason for this is assumed to be destruction of the negative electrode materials caused by their increase and decrease in volume.
On the other hand, unlike the foregoing materials in pure form, the Japanese Patent Laid-Open Publication No. H07-240201 and the Japanese Patent Laid-Open Publication No. H09-63651 disclose negative electrode materials which comprise non-metallic silisides composed of transition elements and inter-metallic compounds including at least one of group 4B elements, P and Sb, and have a crystal structure of one of the CaF2 type, the ZnS type and the AlLiSi type, as negative electrode materials with an improved cycle life property.
Batteries using the negative electrode materials of the non-metallic silisides composed of transition elements disclosed in the Japanese Patent Laid-Open Publication No. H07-240201 have an improved charge/discharge cycle property when compared with lithium metal negative electrode materials (considering the capacity of the batteries according to an embodiment and a comparative example at the first cycle, the fiftieth cycle and the hundredth cycle). However, when compared with a natural graphite negative electrode material, the increase in the capacity of the battery is only about 12%.
The materials disclosed in the Japanese Patent Laid-Open Publication No. H09-63651 have a better charge/discharge cycle property than a Lixe2x80x94Pb alloy negative electrode material (as shown in tests between an embodiment and a comparative example), and have a larger capacity compared with a graphite negative electrode material. However, the discharge capacity decreases significantly, up to the 10xcx9c20th charge/discharge cycles. Even with Mg2Sn, which is considered to be better than any of the other materials, the discharge capacity decreases to approximately 70% of the initial capacity after about the 20th cycle.
Examples of positive electrode active materials for the non-aqueous electrolyte secondary batteries, which are capable of intercalating and de-intercalating lithium ions, include a lithium transition metal composite oxide with high charge/discharge voltage such as LiCoO2, disclosed in the Japanese Patent Laid-Open Publication No. Other materials such as S55-136131, and LiNiO2, disclosed in the U.S. Pat. No. 4,302,518, aim at even a higher capacity. Examples of such positive electrode active materials further include composite oxides comprising a plurality of metallic elements and lithium such as LiyNixCo1xe2x88x92xO2, disclosed in the Japanese Patent Laid-Open Publication No. S63-299056, and LixMyNzOz (M is one of Fe, Co and Ni, and N is one of Ti, V, Cr and Mn) disclosed in the Japanese Patent Laid-Open Publication No. H04-267053.
Active research has been conducted on LiNiO2 since the supply of Ni, its raw material, is stable and inexpensive, and it is expected to achieve a high capacity.
It has been known that with the thus far disclosed positive electrode active materials, especially LiyNixM1xe2x88x92xO2(M is at least one material selected from a group consisting of cobalt (Co), manganese (Mn), chromium (Cr), iron (Fe), vanadium (V), and aluminum (Al); and x is 1xe2x89xa7xxe2x89xa70.5) there are significant differences in charge/discharge capacity between the initial charging (de-intercalation reaction of lithium) and discharging (intercalation reaction of lithium) in the voltage region usually used as a battery (4.3V-2V against Li)( see, for example, A. Rougier et al., Solid State Ionics 90, 83 (1996)). FIG. 2 shows a schematic view of the electric potential behavior at the initial charge and discharge of the positive electrode and the negative electrode of a battery in which composite particle materials with the same theoretical capacity as the foregoing positive electrode materials are used in the negative electrode.
In FIG. 2, (A-B) is the amount of electricity of the positive electrode charged during the first cycle, (B-C) is the discharge capacity of the positive electrode at the first cycle, and (C-A) is the irreversible capacity of the positive electrode. (Axe2x80x2-Bxe2x80x2) is the amount of electricity of the negative electrode charged during the first cycle, which is equal to (A-B) of the positive electrode. (Bxe2x80x2-Cxe2x80x2) is the potential discharge capacity of the negative electrode at the first cycle, and (Cxe2x80x2-Axe2x80x2) is the irreversible capacity of the negative electrode. The potential discharge capacity of the negative electrode at the first cycle (Bxe2x80x2-Cxe2x80x2) is larger than the discharge capacity of the positive electrode at the first cycle (B-C) by the amount of (Cxe2x80x2-D). Therefore, the initial discharge capacity of the battery is determined by the initial discharge capacity of the positive electrode (B-C). In the charge/discharge cycles that follow from the second cycle onwards, a reversible reaction occurs between (B-C) in the positive electrode and (Bxe2x80x2-D) in the negative electrode, which is the same capacity as (B-C). Thus, an amount of lithium corresponding to the capacity of the negative electrode (Cxe2x80x2-D), remains in the negative electrode as xe2x80x9cdead lithiumxe2x80x9d which can not contribute to the charge/discharge reaction of the battery, thereby lowering the capacity of the battery.
When the theoretical capacity of the positive electrode and the negative electrode are adjusted by increasing the amount of active materials in the positive electrode so that the first discharge capacity of the positive electrode and the negative electrode becomes the same after the first charging, the negative electrode is over charged by the amount of (Cxe2x80x2-D) equal to the amount of xe2x80x9cdead lithiumxe2x80x9d in the negative electrode, namely the amount corresponding to the difference between the irreversible capacity of the positive electrode (C-A) and the negative electrode (Cxe2x80x2-Axe2x80x2).
However, the reversible charge capacity of the negative electrode active material is limited. If charging is conducted beyond that limit, lithium metal deposits on the surface of the negative electrode plate. The deposited lithium reacts with the electrolytic solution and becomes inert, thereby lowering the charge/discharge efficiency and thus lowering the cycle life properties.
Conversely, if the negative electrode capacity is significantly larger than the positive electrode capacity, increase of the capacity of the batteries becomes harder due to the excess negative electrode material contained in the negative electrode.
To solve these problems, the Japanese Patent Laid-Open Publication No. H05-62712 discloses a capacity ratio of the positive electrode to the negative electrode. Calculations made in this disclosure are based on the total capacity. However, in actual use, influences of such factors as strength of charging current, charging voltage, and materials used in the positive electrode and the negative electrode are significant. Thus, when a battery is charged slowly (over a long time), just regulating the ratio of the total capacity as disclosed in the Japanese Patent Laid-Open Publication No. H05-62712 is adequate. However, if the speed of charging is important, as it has been in high-speed charging and pulse charging in recent years, the process is inadequate.
The speed of charging is largely influenced by the specific surface area of the materials. Needless to say, a large specific surface area is more advantageous in terms of charging speed, however, if the specific surface area is excessively large, the capacity retention rate deteriorates markedly due to the generation of gas. Thus, the specific surface area needs to be kept within an appropriate range. With regard to this point, for the batteries using carbon material, favorable ranges of the specific surface area are suggested in the Japanese Patent Laid-Open Publication No. H04-242890 and the Japanese Patent Laid-Open Publication No. S63-276873. The ranges are, in the case of the former, 0.5-10 m2/g and the latter, 1.0 m2/g or larger. The Japanese Patent Laid-Open Publication No. H04-249073 and the Japanese Patent Laid-Open Publication No. H06-103976 disclose favorable ranges for the specific surface area of the positive electrode materials, that is, in the case of the former, 0.01-3 m2/g and the latter, 0.5-10 m2/g.
However, when considering a performance of a battery, the balance of intercalation and de-intercalation capacity between the positive electrode and the negative electrode is important, thus merely controlling the capacity of one element separately is meaningless. In other words, regulating the specific surface area of the positive electrode and the negative electrode independently, as has been conducted conventionally, is not satisfactory.
The present invention aims to address the problems of conventional batteries described thus far.
The present invention relates to non-aqueous electrolyte secondary batteries comprising an positive electrode and a negative electrode capable of intercalating and de-intercalating lithium, a non-aqueous electrolyte and separators or solid electrolytes. The negative electrode is characterized by its main material which uses composite particles constructed in such a manner that at least part of the surrounding surface of nuclear particles, containing at least one of tin (Sn), silicon (Si) and zinc (Zn) as a constituent element, is coated with a solid solution or an inter-metallic compound composed of an element included in the nuclear particles and at least one element (exclusive of the elements included in the nuclear particles) selected from transition elements, elements of group 2, group 12, group 13 and group 14 (exclusive of carbon) of the Periodic Table.
To improve the performance of the battery, the composite particles mentioned above can include at least one trace element selected from iron, lead and bismuth. Amounts of the trace element to be added is between 0.0005 wt % and 0.002 wt % or more.
The porosity of the mixture layer at the negative electrode is 10% or more and 50% or less. The porosity of the mixture layer is defined as:
total volume of the space area of the mixture layer/total volume of the mixture layerxc3x97100%.
The present invention maintains the most appropriate amount of the electrolytic solution between the electrode plates by setting it at about 0.1 ml to about 0.4 ml per 1 gram of the total weight of the positive electrode and the negative electrode materials in the battery casing.
The thickness of the separators located in between the positive electrode and the negative electrode of the battery of the present invention is about 15 xcexcm to 40 xcexcm. The piercing strength of the separators is 200 g or more.
Fluorinated carbon compounds defined as (CxF) n (1xe2x89xa6xxe2x89xa620) or metallic compounds which can be reduced electrochemically to metal by charging are added to the negative electrode materials of the battery of the present invention.
Regarding the battery of the present invention, the ratio of (specific surface area of the negative electrode material) to (specific surface area of the positive electrode material) is set at 0.3-12. In the same manner, when R1 is a diameter of a semi-circle arc plotted on a complex plane by measuring impedance at a range of frequencies between 10 kHz and 10 MHz using an electrochemical battery in which an positive electrode plate is set as an active electrode and lithium metal is used in the other electrode; and R2 is a diameter of a semi-circle arc plotted on a complex plane by measuring impedance at a range of frequencies between 10 kHz and 10 MHz using an electrochemical battery in which a negative electrode plate is set as an active electrode and lithium metal is used in the other electrode, the value of R2/R1 is between 0.01-15. Based on the value of R2/R1, the specific surface area of the negative electrode material and the positive electrode material is estimated.
The foregoing construction suppresses an internal short circuit between the positive electrode and the negative electrode caused by expansion of the negative electrode material, thereby providing a high capacity battery with a superior charge/discharge cycle property suitable for a high-speed charging.